49 research outputs found
Out-of-time-ordered measurements as a probe of quantum dynamics
Probing the out-of-equilibrium dynamics of quantum matter has gained renewed
interest owing to immense experimental progress in artifcial quantum systems.
Dynamical quantum measures such as the growth of entanglement entropy (EE) and
out-of-time ordered correlators (OTOCs) have been shown, theoretically, to
provide great insight by exposing subtle quantum features invisible to
traditional measures such as mass transport. However, measuring them in
experiments requires either identical copies of the system, an ancilla qubit
coupled to the whole system, or many measurements on a single copy, thereby
making scalability extremely complex and hence, severely limiting their
potential. Here, we introduce an alternate quantity the out-of-time-ordered
measurement (OTOM) which involves measuring a single observable on a single
copy of the system, while retaining the distinctive features of the OTOCs. We
show, theoretically, that OTOMs are closely related to OTOCs in a doubled
system with the same quantum statistical properties as the original system.
Using exact diagonalization, we numerically simulate classical mass transport,
as well as quantum dynamics through computations of the OTOC, the OTOM, and the
EE in quantum spin chain models in various interesting regimes (including
chaotic and many-body localized systems). Our results demonstrate that an OTOM
can successfully reveal subtle aspects of quantum dynamics hidden to classical
measures, and crucially, provide experimental access to them.Comment: 7 pages, 4 figure
Friedel oscillations due to Fermi arcs in Weyl semimetals
Weyl semimetals harbor unusual surface states known as Fermi arcs, which are
essentially disjoint segments of a two dimensional Fermi surface. We describe a
prescription for obtaining Fermi arcs of arbitrary shape and connectivity by
stacking alternate two dimensional electron and hole Fermi surfaces and adding
suitable interlayer coupling. Using this prescription, we compute the local
density of states -- a quantity directly relevant to scanning tunneling
microscopy -- on a Weyl semimetal surface in the presence of a point scatterer
and present results for a particular model that is expected to apply to
pyrochlore iridate Weyl semimetals. For thin samples, Fermi arcs on opposite
surfaces conspire to allow nested backscattering, resulting in strong Friedel
oscillations on the surface. These oscillations die out as the sample thickness
is increased and Fermi arcs from the bottom surface retreat and weak
oscillations, due to scattering between the top surface Fermi arcs alone,
survive. The surface spectral function -- accessible to photoemission
experiments -- is also computed. In the thermodynamic limit, this calculation
can be done analytically and separate contributions from the Fermi arcs and the
bulk states can be seen.Comment: 5 pages, 2 figures; minor changes in figures and text, typos
correcte
Intrinsic surface superconducting instability in Type-I Weyl Semimetals
Recent experiments on non-magnetic Weyl semimetals have seen separate bulk
and surface superconductivity in Weyl semimetals, which raises the question of
whether the surface Fermi arcs can support intrinsic superconductivity while
the bulk stays in the normal state. A theoretical answer to this question is
hindered by the absence of a well-defined surface Hamiltonian since the Fermi
arcs merge with the bulk states at their endpoints. Using an alternate, Green's
functions-based approach on a phenomenological model that can yield arbitrary
Fermi arcs, we show -- within mean-field theory -- that the surface can support
a standard Cooper instability while the bulk remains disordered. Although the
surface has lower dimensionality, a higher density of states compared to the
bulk allows it to have a higher mean-field superconducting transition
temperature.Comment: 11 pages, 4 figure